This invention relates to control of an oxygen storage amount of an exhaust gas purification catalyst in an internal combustion engine.
Tokkai Hei 10-184424 published by the Japanese Patent Office in 1998 discloses control of the storage amount of oxygen stored in a three-way catalyst in an exhaust passage of an internal combustion engine. This prior art technique calculates a release amount of stored oxygen in the three-way catalyst based on an air/fuel ratio detected by an air/fuel ratio sensor installed upstream of the catalyst in the exhaust passage, an intake air amount of the engine, and a speed ratio of oxygen storage and release of the catalyst. The accuracy of calculating the oxygen storage amount in the three-way catalyst is improved by correcting the speed ratio so that the calculated release amount coincides with the actual release amount.
The prior art technique controls the oxygen amount in the exhaust gas based on the output of the air/fuel ratio sensor in order to maintain the oxygen storage amount of the three-way catalyst to a target range. Specifically, the oxygen amount in the exhaust gas is modified based on the deviation of the oxygen storage amount in the catalyst from the target range. However when the three-way catalyst deteriorates, the oxygen storage capacity is reduced. As a result, if the oxygen storage amount of the three-way catalyst which has not undergone deterioration is controlled on the basis of a gain for the convergence of an oxygen storage amount in a deteriorated catalyst to the target range, long time is required for the convergence of the oxygen storage amount to the target range.
Furthermore the prior art technique sets a target air/fuel ratio of the air/fuel mixture supplied to the engine based on the calculated oxygen storage amount of the three-way catalyst, and controls the oxygen amount per unit time supplied to the engine based on the target air/fuel ratio. In this case, the oxygen amount supplied to the engine varies in response to the intake air amount even when the air/fuel ratio is held constant. Consequently, the oxygen amount per unit time supplied to the engine can not be controlled by only controlling the air/fuel ratio to the target air/fuel ratio.
The prior art technique makes the assumption in the calculation process for the oxygen storage amount of the three-way catalyst that the storage and release of oxygen is performed in a fixed ratio in the three-way catalyst. It is actually the case however that both the release speed and the storage speed of oxygen decrease after the catalyst has stored a certain amount of oxygen. Thus when the release speed and the storage speed of oxygen are respectively fixed, errors tend to result in the calculation of the oxygen storage amount.
It is therefore an object of this invention to increase the speed with which an oxygen storage amount converges to a target range set for a catalyst performing release and storage of oxygen.
It is a further object of this invention to increase the control accuracy on the oxygen amount supplied to the engine.
It is yet a further object of this invention to increase the calculation accuracy for the oxygen storage amount in the catalyst.
In order to achieve the above objects, this invention provides an air/fuel ratio control device for such an engine that is provided with an exhaust gas purification catalyst storing and releasing oxygen in response to the oxygen concentration of exhaust gas of the engine. The device comprises a fuel injector which injects fuel in the engine to generate an air/fuel mixture, a sensor which detects an air/fuel ratio of the air/fuel mixture, a sensor which detects a flowrate of the exhaust gas, and a programmable controller.
The controller is programmed to calculate an oxygen concentration of the exhaust gas based on the air/fuel ratio, calculate an oxygen amount in the exhaust gas based on the oxygen concentration and the flowrate of the exhaust gas, calculate a variation rate of an oxygen storage amount of the catalyst based on the oxygen amount in the exhaust gas, calculate the oxygen storage amount of the catalyst by accumulating the variation rate, calculate a target oxygen amount in the exhaust gas which reduces the difference between the oxygen storage amount and the predetermined target oxygen storage amount, calculate a target air/fuel ratio based on the target oxygen amount and the flowrate of the exhaust gas, and control a fuel injection amount of the fuel injector to a fuel injection amount corresponding to the target air/fuel ratio.
This invention also provides an air/fuel ratio control method for such an engine that is provided with an exhaust gas purification catalyst storing and releasing oxygen in response to the oxygen concentration of exhaust gas of the engine and a fuel injector which injects fuel in the engine to generate an air/fuel mixture.
The method comprises detecting an air/fuel ratio of the air/fuel mixture, detecting a flowrate of the exhaust gas, calculating an oxygen concentration of the exhaust gas based on the air/fuel ratio, calculating an oxygen amount in the exhaust gas based on the oxygen concentration and the flowrate of the exhaust gas, calculating a variation rate of an oxygen storage amount of the catalyst based on the oxygen amount in the exhaust gas, calculating the oxygen storage amount of the catalyst by accumulating the variation rate, calculating a target oxygen amount in the exhaust gas which reduces the difference between the oxygen storage amount and the predetermined target oxygen storage amount, calculating a target air/fuel ratio based on the target oxygen amount and the flowrate of the exhaust gas, and controlling a fuel injection amount of the fuel injector to a fuel injection amount corresponding to the target air/fuel ratio.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.